1. Field of the Invention
The present invention relates to the field of semiconductor processing and more specifically to methods and apparatuses for depositing an oxide film.
2. Discussion of Related Art
Silicon oxide (SiO2) films and their binary and ternary silicates (generally referred to as oxide films) have wide use in fabrication of integrated circuits such as microprocessors and memories. The oxide films are used as insulations between polysilicon layers, metal layers, and metal layers in multilevel metal systems. The oxide films can also be used as diffusion sources, diffusion masks, implantation masks, spacers, and final passivation layers. Currently, there are three different methods of forming an oxide film, dry oxidation (also known as thermal oxidation), steam oxidation, and oxide deposition.
In dry oxidation, an oxide film is grown on a silicon substrate. In one embodiment, the oxide film is grown in a conventional furnace system. FIG. 1 illustrates a furnace system 100 which is a hot wall furnace system including a three-zone resistance furnace 112, a quartz reactor tube 102, a gas inlet 104, a pressure sensor 106, and a wafer boat 108. Multiple silicon wafers 110 are vertically positioned upon the wafer boat 108. The substrates are radiantly heated to a temperature between 1100xc2x0 C. and 1300xc2x0 C. by resistive heating coils surrounding the tube 102. Oxygen (O2) gas is metered into one end of the tube 102 (gas inlet 104) using a mass flow controller. The O2 gas decomposes releasing oxygen (O) atoms; and, the oxygen atoms react with the silicon atoms at the surface of the silicon substrate 110 to form the oxide film. Hours or even days are often needed to form a sufficiently thick oxide film under dry oxidation because after the formation of the first few atomic layers, longer diffusion is required for the oxygen to get to the silicon surface to grow the oxide film. The long hours required to grow the oxide film unnecessarily decreases the throughput for making the oxide film as well as the throughput for making other devices that need the oxide film.
In steam oxidation, an oxide film is also grown on a silicon substrate. The furnace system 100 described in FIG. 1 can also be used. Instead of using an O2 gas, a water vapor (H2O) is introduced into the furnace system 100. With the water vapor, the oxide film can be formed at a lower temperature than in the case of the dry oxidation. A sufficiently thick oxide film can be formed in a shorter amount of time than as required for the case of the dry oxidation. One reason for that is that it is easier to break the oxygen bond in the water vapor than to break the oxygen bond in the O2 gas. However, the oxide film formed under the steam oxidation is less pure than the oxide film formed under the dry oxidation because it is more difficult to get pure water than to get pure O2 gas.
In oxide deposition, an oxide film is deposited onto the surface of a silicon substrate. The furnace system 100 described in FIG. 1 can also be used. A silicon source gas such as silane (SiH4) and an oxidation source gas such as nitrous oxide (NO2) are metered into the end of the tube 102 (gas inlet 104) using mass flow controllers. A temperature between 300xc2x0 C. and 400xc2x0 C. can be used for the oxide deposition. The SiH4 gas and the NO2 gas are decomposed under this temperature. The silicon and the oxygen intermediate species react near the surface of the silicon substrate and attach to the substrate surface to form the oxide film. Depositing the oxide film requires less time than growing the oxide film as in the case of the dry oxidation. However, the deposited oxide film has a much lower film density and quality than the grown oxide film. Additionally, the oxide film tends to contain more impurities because any impurity at the surface of the silicon wafer is trapped at the substrate surface when the oxide film is deposited. Therefore, the oxide film formed by oxide deposition typically has a weaker film interface with the substrate than the oxide film formed by growing.
High quality oxide films are important for continuous advancement in the fabrication of semiconductor devices. High quality oxide films are needed for film stacks such as silicon on insulator (SOI) substrates that are currently in high demand. As illustrated in FIG. 2, a typical SOI substrate 200 includes a silicon substrate 202, an oxide layer 204, and a silicon layer 206. Devices such as transistors and capacitors typically formed on a silicon substrate can be formed on an SOI substrate. SOI substrates are in high demand because they have low current leakage, which allows electronic devices created on the SOI substrates to consume less power. Additionally, the electronic devices created on the SOI substrates can be made smaller.
There are several methods known in the art that can be used to create an SOI substrate. For example, an SOI substrate may be fabricated using a separation by implant oxygen (SIMOX) process, of a bond and etch back (BE) process, a hydrogen implant and release silicon process (sometimes known as SmartCut(copyright)) (xe2x80x9cSmartCut(copyright)xe2x80x9d is a registered trademark of Soitec Silicon on insulator technology S.A.), or by using a plasma implanting oxygen into silicon process.
In any of the SOI forming methods, forming a high quality oxide film with a high throughput is crucial for a making a good SOI substrate.
The embodiments of the present invention relates methods and apparatuses for forming a high quality oxide film that can be used for fabricating a film stack such as a silicon on insulator (SOI) substrate. The oxide film is deposited on a substrate under a high temperature condition wherein the temperature is between 800xc2x0 C. and 1300xc2x0 C., and ideally, between 1000xc2x0 C. and 1200xc2x0 C. To deposit the oxide film, a process gas mixture comprising hydrogen (H2) gas, oxygen (O2) gas, a silicon source gas (e.g., silane, disilane, dichlorosilane, trichlorosilane, tetrachlorosilane, and hexachlorodisilane) is used. A chlorine containing source gas such as HCl or Cl2 may also be used. At all time during the film deposition, the process gas mixture comprises less than 6% oxygen gas, a silicon source gas, and predominantly hydrogen gas. The process gas mixture is thermally decomposed and the oxide film is then deposited on the substrate. The oxide film deposited under such condition is formed in a short period of time, for example, in a matter of minutes. The oxide film further has a high density that is comparable to an oxide film that is formed by the dry oxidation. The oxide film also has substantially no impurity and no impurity trapped on the substrate. The oxide film deposited according to the exemplary embodiment is deposited with a high throughput and thus, allows for efficient fabrication of semiconductor devices that includes the oxide film.